Ribbon feed and correction device for a high speed printer

ABSTRACT

A ribbon correction device aligns a web by deliberately skewing a pair of pivoted winding spools to steer the ribbon continuously toward a given median point along the axis of a spool. The device automatically compensates for the effect of a change in the direction of rotation of the winding spool, by means of a single sensing device.

United States Patent 1 Moneagle et al.

[451 Sept. 13,1973

1 1 RIBBON FEED AND CORRECTION DEVICE FOR A HIGH SPEED PRINTER V [75] Inventors: Otto F. Moneagle; Karl Svatek, both of Endicott, NY.

[73] Assignee: International Business Machines Corporation, Armonk, NY.

[22] Filed: June 23, 1971 [21] Appl. No.: 155,925

[52] US. Cl. 242/57.l [51] Int. Cl. B65h 25/26 Field of Search 242/57.1, 57, 67.3 R,

242/54 R, 55; 226/3, 18, 21, 22, 23; 250/202, 250/219 R, 219 DR, 219 WE [56] v References Cited UNITED STATES PATENTS 3,009,664 11/1961 Huck 242/57.1 2,434,128 1/1948 Stanford 242/57.1 2,654,599 10/1953 Frisbie 271/2.6 2,666,598 1/1954 Robinette 242/76 2,777,069 1/1957 Sueman 250/219 2,883,559 4/1959 Bailey 250/219 2,946,532 7/1960 Carter 242/57.1

3,073,495 1/1963 Evers 226/23 3,096,919 7/1963 Snyder.... 226/3 3,322,963 5/1967 Pages 250/219 3,477,655 11/1969 Anderson 2412/57.] 1,042,905 10/1912 Gamble 74/1255 OTHER PUBLICATIONS IBM Customer Engineering Reference Manual- 1403 Printer, IBM Corporation, Data Processing Division, 1960, 1961, pages 22-26.

Primary Examiner-George F. Mautz Assistant Examiner-Edward J McCarthy Attorney-Hanifin & Jancin [57] ABSTRACT A ribbon correction device aligns a web by deliberately skewing a pair of pivoted winding spools to steer the ribbon continuously toward a given median point along the axis of a spool. The device automatically compensates for the effect of a change in the direction of rotation of the winding spool, by means of a single sensing device.

9 Claims, 14 Drawing Figures mmgnsm 81973 55 sum 1 or 9 OTTOF. MONEA i4 KARL SVATE BYMWf/W ATTORNEY PATENTEU SEFI 8 I973 SHEET 2 7 [IF 9 PATENTEU 3.759.456

SHEET 3 [TE 9 l ROTARY-TO-RECI ROCATING MOTION cON ION MEANS WEB, AXTALEPO I N DETECTION MEANS PATENTED W973 3.759.456

sum 9 OF 9 FIG. 9.

BACKGROUND OF THE INVENTION Broad webs such as a printer ribbon of woven fabric will stretch unevenly during the winding process even if constant tension is applied. Trail-off is the tendency of the web to move across the spool when being wound thereupon. Heretofore, compensation for the consequent axial trail-off of the web was accomplished by one or more of the following methods. (1) An axial displacement of the entire spool was effected to produce a corrective force on the web. (2) A tension idler was brought to bear upon one edge of the web to force the web to migrate in the direction of lesser longitudinal tension. (3) One or both winding spools was pivoted so as to move in the direction of winding thus causing a gradient in the longitudinal tension across the width of the web, the web then'responding by migrating in the direction opposite to the gradient. (4) A pair of steering rollers woluld be positioned along the length of the web and laterally displaced in opposite directions, thus causing an axial gradient in the longitudinal tension in the web, forcing it to move in the direction opposite to the gradient. All of the above techniques have the disadvantage of periodically imposing high stresses and strains in the fabric of the web, thus'shortening the life of the ribbon. Other approaches to trail-off correction have included auxiliary, pivoted, steering roller mechanisms in addition to the winding spools. But in every case the mechanical mechanism for effecting the trailoff correction requires a large number of parts and employs electronic or hydraulic sensing means for detecting the axial position of the web, and for instituting the mechanical displacement necessary to correct, therefor. Electronic or hydraulic sensors are costly and space consuming, with their associated power supplies, fluid reservoirs, etc. In electronic detection of the web position where photodetectors are employed, one disadvantage is the tendency of such photodetectors to accumulate ink and paper debris, thus causing them to erroneously signal the presence of a printer ribbon and in fact initiate an erroneous correction.

The high speed printers employed as input/output equipment for computers, run at speeds of 2,000 plus printed lines per minute. The ribbons employed in such printers are as wide as the document printed, and must move longitudinally across the print line at speeds comparable to that of the document itself. Such ribbons,'repeatedly'exposed to high impact hammer blows during the course of the printing operation, have a wear rate which exceeds that in less demanding applications. Where trail-off correction techniques are employed, which induce tension gradients across the width of the ribbon in order to correct for lateral displacements, the wear rate of the ribbon increases.

Upon reversal, the direction of traversal of the web along the spool reverses. This behavior is called a 180 phase shift in the trail-off direction. The prior art teachesthat detection mechanisms for correction of traversal under this condition must be duplicated, one at each end of the spool. Heretofore, solution of this problem was accomplished by complex electronic apparatus employing a multiplicity of photoelectric sensors. Differential tensioning techniques have also been employed with their attendant high wear producing characteristics. Although the prior art devices are adequate for general purpose spooling of broad webs, no simple, reliable device having a small number of parts, has yet been devised to compensate for the trail-off in bi-directional winding applications such as the spooling of high speed printer ribbons.

OBJECTS OF THE INVENTION An object of the invention is to provide a simplified A SUMMARY OF THE INVENTION The trail-off of the broad ribbon web in a high speed printer is simply and effectively compensated for by the subject invention, by means of controllably changing the state of relative skew between the play-out spool and the take-up spool. The play-out spool and the takeup spool are pivotally and rotatably mounted in a twistable frame such that alternate states of twist in the frame will impart alternate states of relative skew between the spools. Each of the two spools can alternately assume the role of take-up spool or play-out spool. The spools are rotatably driven by a motor, and an end of web reversing means reverses the direction of drive of the motor when the end of the web is detected before playing off either spool. The twist which imparts the trail-off correction is brought about by a rotary-to-reciprocating motion conversion means which is intermittently driven by an intermittent clutch means at such times when a web axial-position detection means senses a change in the axial position of the web as it is wound upon either the first or the second spool. The web is thereby steered into alternate directions of axial motion as the take-up spool and the playout spool assume their alternate states of relative skew, as is controlled by the web axial-position detection means. By means of the coaction of these elements, the printer ribbon web may be evenly wound on either spool, automatically. 7

The invention accomplishes the web axial trail-off correction function in an improved manner by not imparting a gradient in the longitudinal tension across the web, thus conferring a longer life to printer ribbons. Furthermore, the axial trail-off correction function is accomplished by the invention with a smaller number of mechanical parts than has heretofore been accomplished by prior art devices. In addition, the invention accomplishes the trail-off correction function automatically, without regard to changes in the winding direction.

DESCRIPTION OF THE DRAWINGS FIG. 1 is a simplified diagram of the relative position of the first and second winding spools during botha winding and unwinding operation.

FIG. 1a through ld show the preferred web winding configuration.

FIG. 1e shows an alternate web winding configuration.

FIG. 2 is a detailed drawing indicating the relative position of the spools, correction cam, and sensing arm.

FIG. 3 is a semi-schematic diagram depicting the working relationship of the elements of the invention.

FIG. 4 depicts in detail the relative positions of the double pawl clutch, correction cam, sensing cam, sensing arm, web and lower spool.

FIG. 5 is a timing diagram indicating how the trail-off correction is effected for counter-clockwise motion of the lower spool.

FIG. 6 is a timing diagram indicating how the trail-off direction is effected during clockwise rotation of the lower spool.

FIG. 7 is a timing diagram indicating how the 180 phase shift correction is effected when the edge of the web is not at the reference point.

FIG. 8. is a timing diagram indicating how the simultaneous 180 phase shift correction and the web trailoff correction are effected when the edge of the web is at the reference point at the time of the change in direction of rotation.

FIG. 9 is an exploded view of the elements in the ribbon correction device.

DESCRIPTION OF THE PREFERRED EMBODIMENT The trail-off correction is accomplished by imparting a relative skew between the take-up and the play-off spools. To induce this skew and to control its magnitude and direction, a twistable frame means mounted to the machine frame, is employed. The spools are pivotally and rotatably mounted in a first base plate which is rigidly attached and a second base plate which is pivotally mounted to the machine frame. The pivot point for the second base plate is located substantially midway between the mounting position for the first and second spools. Thus when the second base plate is pivoted about the pivot point, the adjacent end of the first spool is displaced in a forward direction by substantially the same distance as the displacement of the corresponding end of the second spool in the backward direction. Thus there is substantially no difference in the longitudinal stress in the web between that for the extreme forward skew and that for the extreme reverse skew of the two spools.

FIG. 1 is a simplified diagram showing the relative position of the spools during the trail-off correction sequence. I-Iaving reference to FIG. 1, upper spool 2 and lower spool 4 are rotatably and pivotally mounted on fixed plate 6 and are rotatably and pivotally mounted in pivoted plate 8 such that when plate 8 is rotated about the pivot 10, spools 2 and 4 assume a skew position with respect to one another. The web 16 is wound about the upper spool 2 in a clockwise direction and proceeds thence over backing surface 26 and is then wound in counterclockwise fashion about the lower spool 4. In such configuration, when the ribbon is moving from upper spool 2 to lower spool 4 and the ribbon is not present at the reference point under the sensing finger 20, the upper spool 2 is pivoted away from the print line 18 by equal displacements such that the ribbon is caused to be steered toward the observer such that it will subsequently migrate across the imaginary reference point beneath the sensing finger 20. When this transition condition obtains, the sensing finger 20 falls, causing the sensing arm to undergo a relatively large displacement. At this time of transition, the pivoted plate 8 is rotated about the pivot 10 such that the upper spool 2 is skewed toward the print line and the lower spool 4 is skewed away from the print line as in FIG. lb. This displacement, after the ribbon overcomes its inertia in the original direction of motion, effects the desired change in the direction of motion, away from the observer.

In FIG. 10, when the ribbon is moving from the lower spool 4 to the upper spool 2 and the ribbon is under the sensing finger 20, the upper spool 2 will be away from the print line 18 and the lower spool 4 will be toward the print line 18 such that the preferred direction of trail-off motion is away from the observer. When the transition point occurs, where the ribbon passes from beneath the reference point, a corrective skew displacement takes place. The upper spool 2 moves toward the print line 18 and the lower spool 4 moves away from the print line 18 such that the ribbon will tend now to be steered toward the observer, as in FIG. 1d.

The preferred embodiment for the wound web configuration is that shown in FIGS. la through d, that is where the web winds off of the first spool and onto the second spool at those respective spool surfaces closest to one another. FIG. 1c shows an alternate embodiment where the web is wound off of the first spool and onto the second spool at those respective spool surfaces which are at the extreme distances from one another. The trail-off correction sequence for both configurations, however, will be the same since the relative direction of rotation of the upper and lower spools at any one time is always the same for each of these configurations.

FIG. 2 shows an end view of the twistable'frame means and the relative positions of the web, the web axial-position detection means, the end-of-web reversing means, the driving means and the rotary-toreciprocating motion conversion means. The web, wound between the upper and lower spools, is periodically sampled by the web axial-position detection means for its presence or absence at a reference point midway between the extreme axial displacements of the web. When the edge of the web traverses the reference point, the amplitude of the periodic displacements of the detection means changes, ultimately causing the rotary-to-reciprocating motion conversion means to displace the twistable frame means into its alternate state of twist. The state of relative skew between the first and second spools is thereby changed imparting the desired trailoff correction to the axial motion of the web.

Referring to FIG. 2, the lower spool 4, onto which is wound the web 16, is concentric with the shaft 36 for the correction cam 32. The sensing arm 38 whose oscillations are derived from the rotary motion of shaft 36, causes the sensing finger 20b to periodically pass through the reference point 40 such that when the web is absent from reference point 40, the sensing finger will remain in the standing position 201). However, when the web is present over reference point 40, the sensing finger will assume the fallen position 200. The consequent change in the magnitude of the displacement of the sensing arm 38 institutes a rotation of correction cam 32 effecting the displacement of the pivot plate 8 about pivot thus effecting the change in the relative skew of spools 2 and 4.

FIG. 2 shows the relative position of the ribbon reversing arm 46, motor reversing switch 50, the motor for upper spool 2 and the motor for lower spool 4. The ribbon drive is accomplished by two motors designated upper 112 and lower 110. While one motor is driving or winding, a limited voltage is provided to the opposite motor to control the ribbon tension. The ribbon only moves during printing and stops approximately 120 ms after the end of printing, at which time one half of the voltage is supplied to both ribbon drive motors, urging them in opposite directions to maintain tension in the ribbon. The ribbon reversing function is accomplished by the pivoted reversing arm 46 which is shifted by reversing bars attached to each end of the ribbon near the point were the ribbon attaches to the respective spools 2 and 4. When the arm 46 is operated, it in turn actuates motor reversing switch 50 that determines which motor will wind the ribbon in the proper direction. The ribbon reversing arm 48 is pivoted at 52.

Turning now to FIG. 3, the semi-schematic diagram depicts the working relationship of the various elements of the invention. The sectional view is taken along the axis of pivot 10 in FIG. 1. Mounted on machine frame 120 is fixed base plate 6 and pivoted base plate 8, together which make up the twistable frame means. The pivoted base plate 8 can undergo angular displacements with respect to the machine frame 120, about the pivot 10. The lower spool shaft 36 is mounted in fixed base plate 6 through a universal coupling 122 and is mounted in pivoted base plate 8 by universal coupling 124 such that said twistable frame means can undergo twistdisplacements without binding the shaft 36. Driving means 110 is mounted to the twistable frame means and drives shaft 36 through universal coupling 122. Mounted on shaft 36 is the lower spooling means 4, the sensing cam 60, and the ratchet 54, each of which is continuously driven by the motor 110. The sensing arm 38, pivotally mounted on the twistable frame means, is periodically driven by the sensing cam 60 through the sensing cam follower 66. Sensing finger pivotally mounted on the end of sensing arm 38, is driven into periodic engagement with the moving web. Correction cam shaft 36a, rotatably mounted on the twistable frame means and not directly connected to the lower spool shaft 36, carries the pawl carrier 86 and the correction cam 32, which rotate in unison with the correction cam shaft 36a. Pivotally mounted on pawl carrier 86 are pawls 90 and 94. Link lever 72 connected with sensing arm 38, periodically actuates pawls 90 and 94 in timed relation with the rotation of ratchet 54. When a change takes place in the magnitude of the periodic displacements of sensing arm 38, due to the detection of a change in the position of the edge of the web, link lever 72 causes pawl 90 or 94 to engage ratchet 54. Pawl carrier 86 is thereby caused to rotate with the engaged ratchet until the detent 104 causes the-pawl carrier 86 to cease its rotation after 180, at which time link lever 72 causes the engaged pawl 90 or 94, to disengage the ratchet 54. The correction cam follower 34, within which the correction cam 32 rotates, is rigidly fixed to the machine frame. When the correction cam 32 rotates within cam follower 34, the correction cam shaft 36a, and therewith the twistable frame means, undergoesa lateral displacement with respect to the machine frame, base plate 8 pivoting about pivot 10. Since correction cam 32 rotates in unison with pawl carrier 86, when sensing arm 38 causes the pawls or 94 to engage ratchet. 54, pawl carrier 86 thereby undergoes rotation causing correction cam 32 to rotate within correction cam follower 34 by 180. This induces the desired twist displacement in the twistable frame means. The change in the relative skew between the upper and the lower spooling means induced by the twist displacement in the twistable frame means, imparts the necessary lateral steering correction to the web as it winds upon either the upper or the lower spool. In order to clarify the description of the working relationship between the elements, FIG. 3 has been simplified with respect to the relative position of the parts in the preferred embodiment. In the preferred embodiment, the driving means is mounted on the pivoted base plate 8 and the correction cam shaft 36a is hollow, the lower spool shaft 36 passing therethrough and directly connected with the driving means 110. The sensing arm 38, sensing cam follower 66, and link lever 72 together comprise the web, axial-position de tection means. The ratchet 54, the pawl carrier 86 and its associated pawls 90 and 94, together with the detent 104 comprise the intermittent clutch means. The correction cam 32 and the correction cam follower 34 together comprise the rotary-to-reciprocating motion conversion means. The combination of the intermittent clutch means and the rotary-to-reciprocating motion conversion means constitutes the reciprocating drive means.

FIG. 4 shows the elements comprising the web axialposition detection means, the intermittent clutch means, and the rotary-to-reciprocating motion conversion means.

The web axial-position detection means comprises the sensing cam 60, a sensing arm means, and the sensing finger 20. The sensing arm means comprises the sensing arm 38, link lever 72, spring 34, and lever 82.

The intermittent clutch means comprises the ratchet 54, pawl carrier 86, pawls 90 and 94, and detent 104.

The rotary-to-reciprocating motion conversion means comprises the correction cam 32 and the correction cam follower 34. i

The web axial-position detection means senses the traversal of the edge of the web across the reference point 40, and at such time undergoes a change in the amplitude of its periodic displacements causing the engagement and rotation of the intermittent clutch means. Since the pawl carrier of the clutch means and the correction cam of the rotary-to-reciprocating motion conversion means are mounted together, they rotate in unison and thus impart the desired controlled displacement to the twistable frame means.

Referring to FIG. 4, shaft 36, which is the shaft of the drive motor which is mounted to the pivot plate 8 and which drives the lower spool 4, drives the ratchet 54 and sensing cam 60. Ratchet 54 onto which are mounted in diametric opposition teeth 56 and 58, is fixedly attached to the shaft 36 and fixedly attached to sensing cam 60. Sensing cam 60 has the lobes 62 and 64 diametrically opposed to one another and aligned with the the teeth 56 and 58 of ratchet 54. The lobes 62 and 64 on sensing cam 60 periodically engage the sensing cam follower 66. Sensing cam follower 66 is attached by pin 68 to the sensing arm 38. Sensing arm 38 is pivoted on stud 70 which in turn is mounted on pivot plate 8. Sensing finger 20 mounted on sensing arm 38 and loaded by spring 30 is periodically displaced through the reference point 40. When the web 16, which is drawn across the backing surface 26, covers the reference point 40, the sensing finger collapses thus causing the sensing arm 38 to traverse a larger magnitude displacement than it does when web 16 is absent from reference point 40. The periodic displacements of sensing arm 38 are coupled to link lever 72 through the pin 68. Link lever 72 is biased by spring 74 so that sensing cam follower 66 will always engage sensing cam 60. Link lever 72 is connected to lever 82 through pin 80 and lever 82 is pivoted about stud 84 which in turn is anchored to the plate 8. When lobes 62 or 64 of sensing cam 60 engage sensing cam follower 66, link lever 72 is displaced so as to effect the alternate engagement and disengagement of pawls 90 and 94. Pawl carrier 86 is fixedly attached to correction cam 32. Correction cam 32 engages the surface 88 of correction cam follower 34 which is rigidly attached to the machine frame. Thus, when correction cam 32 rotates within the surface 88 of correction cam follower 34, the shaft 36 is laterally displaced with respect to the machine frame. Mounted on pawl carrier 86 is counter- '-clockwise pawl 90 which pivots on pin 92 and clockwise pawl 94 which pivots on pin 96. Pawls 90 and 94 are spring loaded in the engagement position by pawl spring 98. Detent pins 100 and 102 engage the detent 104 at 180 intervals on the pawl carrier 86. The assemblage of pawls 90 and 94 mounted on pawl carrier 86, with the detent 104 and ratchet 54, comprises a double pawl clutch. When the sensing arm 38 undergoes a large displacement, that is, when web 16 covers the reference point 40, link lever 72 also undergoes a larger displacement. Lobe 106 on link lever 72 will engage the tail of pawl 90 when sensing arm 38 undergoes its larger displacement. Thus during counterclockwise rotation when the web is present and the sensing arm has undergone a larger displacement, counterclockwise pawl 90 will not engage the teeth 56 or 58 of ratchet 54 as it rotates with the spool 4. However, when the transition point occurs at which web 16 traverses reference point 40 and no longer covers reference point 40, sensing finger 20 will remain at a standing position preventing sensing arm 38 from undergoing its larger displacement. Thus, lobe 106 of link lever 72 will not contact the tail of counterclockwise pawl 90. Thus as one of the teeth 56 or 58 of ratchet 54 rotates into engagement position as the ratchet rotates in its counterclockwise direction, that tooth will engage counterclockwise pawl 90 causing pawl carrier 86 and the attached correction cam 32 to rotate in unison with the ratchet 54, in a counterclockwise direction for 180 until the tail of counterclockwise pawl 90 engages the lobe 108 of link lever 72, thus causing the disengagement of pawl 90 from the tooth of ratchet 54. The motion of pawl carrier 86 is arrested by the engagement of detent pin 100 or 102 with detent 104. The rotation of the correction cam 32 through 180 will have, by virtue of its con tact with the surface 88 of correction cam follower 34, caused the lateral displacement or skewing of the shaft 36 with respect to the machine frame. Because shaft 36 is rotatably mounted in pivot plate 8, pivot plate 8 will have pivoted about its pivot point thus causing a corresponding displacement in the opposite direction for the upper spool. A graphic explanation of the interaction of sensing arm 38, clockwise pawl 94 and counterclockwise pawl 90 and of the relation between the corrective displacement of shaft 36 and the position of the web with respect to reference point 40, can be gained by studying the timing diagram shown in FIG. 5.

OPERATION The objective of the operation of the invention is to impart a correction to the trail-off direction of the web as it traverses a given median point in the axial direction. To accomplish this the web axial-displacement detection means undergoes a periodic displacement through the reference point. When the web is present the periodic displacements are larger than when the web is absent from the reference point. When the edge of the web traverses the reference point, the consequent change in the amplitude of the periodic displacements triggers the intermittent clutch means to engage, thereby rotating the correction cam in the rotary-toreciprocating motion conversion means. The motion conversion means induces a twist displacement in the twistable frame means, and thereby a change in the relative skew of the first and second spools upon which the web is wound. The web is thereby steered such that the direction of axial trail-off is reversed, the net eflect of which is to accomplish an even winding of the web onto either spool.

With reference to FIG. 4, when the lower spool 4 is undergoing clockwise rotation, it is clockwise pawl 94 which alternately engages and disengages the teeth of ratchet 54, thus effecting the controlled displacement of shaft 36. The timing diagram depicted in FIG. 6 is of assistance in understanding the sequence of events which effect the trail-off correction during clockwise rotation. When the web 16 does not appear beneath the reference point 40, sensing arm 38, as urged by sensing cam 62s actuation of cam follower 66, undergoes a small displacement Y. Clockwise pawl 94 is, and remains in, the open position and counterclockwise pawl remains in a closed position. Correction cam is so placed so that the shaft 36 is displaced back from the print line by the distance X. This corresponds to the period P1 in FIG. 6. At the time :5, the transition time, web 16 traverses reference point 40, sensing finger 20 collapses causing sensing arm 38 to undergo a larger displacement Y, link lever 72 causes lobe 108 to release the clockwise pawl 94, the clockwise rotation of ratchet 54 causes one of its teeth 56 or 58 to engage the clockwise pawl thus initiating the rotation of the pawl carrier 86 and thus the rotation of the correction cam 32. The clockwise rotation of correction cam 32 continues for 180 of rotation until the tail of clockwise pawl 94 engages the lobe 106 of link lever 72. At this point, clockwise pawl 94 is disengaged from the tooth of ratchet 54 and the detent 104 simultaneously engages detent pin or 102 thus arresting the rotation of the correction cam 32. At this point the shaft 36 has been displaced by a distance X toward the print line, that is, to the front, and the period P2 in FIG. 6 has commenced. During this period the web will continue to cover the reference point 40 thus causing the sensing arm stroke Y to be large. At every stoke of the sensing arm 38, lobe 106 of link lever 72 engages clockwise pawl 94 just prior to the approach of the tooth 56 or 58 of ratchet 54, thus avoiding engagement and rotation when no correction is needed. When, as at time I13 in FIG. 6, the transition point occurs where web 16 traverses reference point 40 from beneath the finger to outside the region of the sensing finger 20, sensing finger 20 will remain standing. As a result, the periodic oscillations of sensing arm 39 are of smaller magnitude. During the first rotation of the tooth 56 or 58 of ratchet 54 to the point of potential engagement with clockwise pawl 94, the tail of clockwise pawl 94 is no longer engaged by lobe 106 of link lever 72. Thus, pawl 94 engages tooth 56 or 58 of ratchet 54 and again a 180 rotation of pawl carrier 86, and thus, of correction cam 32, is effected. This action is depicted at times t14 to r16 in FIG. 6. Shaft 36 is seen to be displaced back to its original position of magnitude X and is now farther away from the print line than is the axis of the upper spool.

FIG. 7 illustrates the sequence of events that occurs when the direction of rotation of the spools is reversed at a time when the web is not at a transition point. Period P1 in FIG. 7 is identical to period P1 in FIG. and period P2 in FIG. 7 is identical to period P2 in FIG. 5 up to the time tl2. At time :12 the direction of rotation of the lower spool is reversed from counterclockwise to clockwise, as is the direction of rotation of the upper spool. The direction of motion of the web can be seen to immediately reverse, illustrating the 180 phase shift phenomenon at time tl2. Clockwise pawl 94 engages tooth 56 or 58 of ratchet 54 in the first stage of the commencement of clockwise rotation of ratchet 54. Clockwise pawl 94 remains engaged with ratchet 54 until pawl carrier 86 has rotated 180 at which time clockwise pawl 94 engages lobe 106 of link lever 72. Recalling that the web is beneath sensing finger 20, sensing arm 38 is undergoing large displacements thus, lobe 106 will be in position to disengage clockwise pawl 94 for every 180 rotation of ratchet 54 until the web undergoes a transition point at t 19. It is seen that from tl4 on through r35 in FIG. 7, the sequence of events is identical to that in normal clockwise rotation of spool 4, as is shown in FIG. 6. Thus, 180 phase shift correction upon a change in the direction of rotation of the v spool, is accomplished automatically by virtue of the interaction of the double pawl clutch assembly, the sensing cam, and the sensing arm.

FIG. 8 illustrates the sequence of events that occur when the direction of rotation of the spools is reversed at a time when the edge of the web is traversing reference point 40. Periods P1 and P2 in FIG. 8 are identical to periods P1 and P2 in FIG. 5 and period P3 in FIG. 8 is identical to period P3 in FIG. 5 up to the time :14. At time :14, the direction of rotation of the lower spool is reversed from counterclockwise to clockwise, as is the direction of rotation of the upper spool. The direction of axial motion of the web can be seen to immediately reverse. Since the web has justpassed the transition point at tl3, a trail-off correction would normally have gone into effect at time :14 so that the web could be steered back toward the reference point 40. However, by reversing the direction of rotation of spool 4, the normal trail-off correction becomes unnecessary and indeed if made at this time, would cause the web to spiral off uncontrollably along the axis of the spool. However, the interaction of the double pawl clutch, the sensing cam and the sensing arm, cause a suppression of the 180 phase shift correction, which commences at r14 and remains in effect until the edge of the web is sensed to have once again traversed reference point 40, at which time the normal clockwise sequence of events commences, as is shown in FIG. 6. From time t] to time tl4 in FIG. 8, the normal clockwise sequence of events has taken place. Had spool reversal not occurred at time :14, the smaller magnitude displacement Y of sensing arm 38 would have prevented the engagement of lobe 106 of link lever 72 with the tail of counter clockwise pawl 90, thus causing the normal engagement with ratchet 54 resulting in the corrective rota tion of correction cam 32. However, since the direction of spool rotation was reversed to clockwise at time :14, the counterclockwise pawl is no longer effective. If, at the time of spool reversal, the edge of the web 16 had not been traversing reference point 40, but was still beneath the sensing finger 20 as is shown in FIG. 7, the larger magnitude displacement Y of sensing arm 38 would have prevented lobe 108 of link lever 72 from engaging the tail of clockwise pawl 94; thus permitting the clockwise pawl to engage the ratchet 54, thus causing the 180 phase shift correction to commence, as would normally have taken place. However, where spool reversal takes place at or immediately after the time at which the edge of the web 16 traverses the reference point 40, neither of the foregoing corrective sequences should take place. To accommodate this requirement, at time 114 in FIG. 8, sensing arm 38 commences its smaller magnitude displacements, causing lobe 108 of link lever 72 to contact the disengage clockwise pawl 94 from its potential engagement position with ratchet 54. Clockwise pawl 94 remains in the disengaged position during the period required for the edge of web 16 to return to reference point 40 under the influence of the reversed direction of rotation of spool 4. After the edge of web 16 is detected by sensing finger 20, to have traversed reference point 40, which occurs at time tl6 in FIG. 8, sensing arm 38 recommences its larger magnitude displacements causing lobe 108 to link lever 72 to disengage the tail of clockwise pawl 94, at which time ratchet 54 is engaged and correction cam 32 is rotated for a normal clockwise trail off correction. After time 216 the nomal clockwise correction sequence obtains as is shown in FIG. 6. The period of time between [14 and r16 in FIG. 8, is the period for suppression of the 180 phase correction.

Referring now to FIG. 9 the relative placement of the various parts previously described, is shown. The motor 110 which drives lower spool 4, is mounted on pivot plate 8. Motor 112 also mounted on plate 8 drives the upper spool. Pivot plate 8 rotates about pivot 10 which is affixed to the machine frame. Sensing cam 60 and ratchet 54 are fixedly attached together and are in turn fixedly attached to shaft 36. Pawl carrier 86 to which is fixedly attached correction cam 32, are rotatably mounted on shaft 36. Correction cam follower 88 is mounted at points 114 and 116 to the machine frame and thus serves as the fixed reference point for the lateral displacement of shaft 36. Sensing cam follower 66, mounted on sensing arm 38, is driven by sensing cam 60 such that sensing finger 20 periodically engages the web, when present. Link lever 72, attached to sensing arm 38, oscillates as the sensing arm is actuated by the sensing cam, and successively actuates clockwise pawl 94 and counterclockwise pawl 90 as the shaft 36 rotates. The state of relative skew between the spools, as mechanically controlled by the position of the web, causes the web to be evenly wound upon the spools, regardless of the direction of rotation.

FIG.9 depicts the ribbon dirve and sense system. The ribbon drive and sense system consists of three functions: the ribbon drive, ribbon reversing and ribbon skew correction.

The ribbon drive is accomplished by two motors designated upper 112 and lower 110. While one motor is driving or winding a limited voltage is provided to the opposite motor to control the ribbon tension. The ribbon only moves during printing and stops approximately 120 ms after the end of printing. At this time, one half voltage is supplied to both ribbon drive motors to maintain tension.

The ribbon reversing function is accomplished by a pivoted reversing arm 42 which is shifted by reversing bars attached to each end of the ribbon near the point where the ribbon attaches to the spools. When the arm is operated, it in turn operates a switch 50 that deterl mines the motor that will wind the ribbon in the proper direction. The ribbon reversing arm is pivoted at 52.

The ribbon skew correction is accomplished by pivoting the ribbon drive unit to change the line of ribbon winding in relation to the direction of ribbon feed. The operation is controlled by the sensing finger 20 and arm 38. The sensing finger mechanically checks to see if the moving ribbon is beneath it. If the ribbon is under the arm, the movement of the ribbon causes the finger to pivot which in turn allows the arm to drop. This sets the drive pawl 94 or 90 which rotates the correction cam 32 180 to pivot the twistable frame means. There are two pawls, one drives when the lower ribbon spool is winding, the other drives when the lower spool is unwinding. The twistable frame means stays in the pivoted position as long as the finger continues to sense the ribbon. Once it is determined that the ribbon has shifted from under the finger, the sensing arm again sets the drive pawl and rotates the twistable frame means back to the starting position by turning the correction cam 180. A sensing cam 60 is provided to raise the finger, via the sensing arm, twice each spool revolution, to permit the ribbon to feed under the finger eliminating the possibility of the ribbon rolling up against the finger instead of moving under it.

While the invention has been particularly shown and described with reference to preferred embodiments thereof, it will be understood by those skilled in the art that the foregoing and other changes in form and details may be made therein without departing from the spirit and the scope of the invention.

We claim:

1. In combination with a device for evenly winding a web onto a spool of the type comprising a fixed machine frame, a twistable frame means mounted on said machine frame, said twistable frame means capable of assuming either of two states of twist, a first and a secnd spooling means, said first and second spooling means pivotally and rotatably mounted in said twistable frame means, said two states of twist imparting alternate states of relative skew between said first and said second spooling means, said first spooling means winding said web from said second spooling means and said second spooling means winding said web from said first spooling means, a rotary driving means for rotatably driving said first and second spooling means, a web axial-position detection means, a reciprocating drive means, said reciprocating drive means intermittently actuated by said detection means when a change is de- 65 tected in the axial-position of said web as it is wound upon either said first or said second spooling means, said twistable frame means driven by said reciprocating drive means so as to assume either of said two states of twist, said web steered into alternate directions of axial motion on said first or said second spooling means by said alternate states of relative skew, said state of skew 5 controlled by said detecting means sensing the presence or absence of said web at a reference point substantially midway between the extreme axial displacements of said web, wherein the improvement resides in said reciprocating drive means which further com prises:

a ratchet means continuously driven by said rotary driving means, a pawl means opposed to said ratchet means and intermittently engagable therewith under the control 5 of said web axial-position detection means,

a rotary-to-reciprocating motion conversion means connected to said pawl means, said motion conversion means being intermittently driven by said pawl means when said detection means causes said pawl means to engage said ratchet means,

said rotary-to-reciprocating motion conversion means contacting to said machine frame and said twistable frame means so as to impart a relative displacement therebetween when driven by said pawl means.

2. In the improved device for evenly winding a web onto a spool of claim 1, wherein said web axial-position detection means further comprises:

a sensing cam, rotatably mounted on said twistable frame means and continuously driven by said rotary driving means,

said sensing cam having at least one lobe,

a sensing arm, mounted on said twistable frame means and resiliently urged into engagement with said sensing cam, said sensing arm periodically displaced by the lobes on said cam, through an are over said reference point,

said sensing arm undergoing relatively large displacements when said web is present and relatively small displacements when said web is absent from said reference point,

3. In the improved device for evenly winding a web onto a spool of claim 2, wherein said reciprocating drive means further comprises:

said ratchet means, rotatably mounted on said twistable frame means and continuously driven by said rotary driving means,

said ratchet means possessing a number of teeth equal to the number of lobes on said sensing cam,

said ratchet means rotating in unison with said sensing cam,

a pawl carrier means,

said carrier means rotatably mounted on said twistable frame means in a position coaxial with and opposed to said ratchet means,

a first pawl, pivotally mounted on said carrier means,

said first pawl mounted so as to be resiliently urged into engagement with said ratchet means when said ratchet means undergoes clockwise rotation,

a second pawl, pivotally mounted on said carrier means,

said second pawl, mounted so as to be resiliently urged into engagement with said ratchet means when said ratchet means undergoes counterclockwise rotation,

a resilient detent means mounted on said twistable frame means and positioned so as to stop the rotation of said pawl carrier means at every 180,

said sensing arm periodically contacting and pivot/ally displacing said first and said second pawls, as it is driven by said sensing cam in timed rotation with the rotation of said ratchet means,

said pawl displacement causing said first pawl to avoid engagement with said ratchet means during clockwise rotation of said ratchet and causing said second pawl to avoid engagement with said ratchet during counterclockwise rotation of said ratchet,

said sensing arm omitting contact with said first pawl during clockwise rotation of said ratchet or omitting contact with said second pawl during counterclockwise rotation of said ratchet, when a change in the magnitude of said periodic displacements of said sensing arm occurs,

said displaced pawl engaging said ratchet means when said change in the magnitude of said sensing arm displacements takes place, said pawl carrier means thereby being driven into rotation,

said pawl carrier rotation continuing until said detent means halts said rotation, at which point said sensing arm contacts said engaged pawl, disengaging said pawl,

said pawl carrier means remaining stationary in its new state as long as said periodic sensing arm displacements retain their new magnitude.

4. In the improved device for evenly winding a web onto a spool of claim 3 wherein said reciprocating drive means further comprises:

a correction cam rotatably mounted on said twistable frame means and rotating in unison with said pawl carrier means, I i

a cam follower rigidly attached to said machine frame,

said correction cam rotatably mounted within said cam follower,

said twistable frame means undergoing a twist displacement when said correction cam is rotatably driven by 180 within said cam follower by means of the controlled rotation of said pawl carrier means,

said state of relative skew of said first and second spooling means being altered by the controlled displacement of said twistable frame means,

said web being steered into alternate directions of axial motion along said first and second spooling means, by the controlled changes in said relative skew,

whereby said web may be evenly wound on either said first or said second spooling means, automatically.

5. In the improved device for evenly winding a web onto a spool of claim 1, wherein the rotary driving means further comprises:

a first reversible motor mounted on said twistable frame means,

said first motor driving said first spooling means,

a second reversible motor mounted on said twistable frame means,

said second motor driving said second spooling means,

said first motor, when driving said first spooling means so as to wind said web thereupon, being fully energized to advance the web, while said second motor being energized in the direction opposing the advance of the web, at a magnitude of substantially one half that of said first motor, thus imparting a drag force on the web,

an end-of-web reversing means for detecting the end of the web before it unwinds from the spooling means supplying said web, said reversing means automatically reversing the driving direction of said first and second motors, said second motor becoming fully energized so as to advance the web in the direction reversed to its preceding motion and said first motor becoming energized at one half its preceding magnitude and in the direction opposed to the new direction of advance for the web, whereby said web remains in tension without regard to its direction of advance.

6. In the improved device for evenly winding a web onto a spool of claim ll, wherein the web axial-position detection means further comprises:

a sensing cam continuously driven by said rotary driving means,

a sensing arm mounted on said twistable frame means so as to be resiliently urged into engagement with said sensing cam,

said sensing arm being periodically lifted and lowered over said reference point by means of said sensing cam,

a collapsible finger m eans, pivota1ly mounted on said sensing arm,

said finger means located substantially over said reference point,

said finger means frictionally engaging said web when present at said reference point, when said cam lowers said sensing arm,

said finger means following the advancing motion of said web, when frictionally engaging the web causing the finger means to collapse and the sensing arm to undergo a relatively large displacement,

said finger means remaining in a standing position on a supporting surface for said web, when said web is absent from said reference point, when said sensing cam lowers said sensing arm, thereby limiting the sensing arm to a relatively small displacement,

said sensing arm periodically actuating said pawl means as said sensing cam lifts and lowers said sensing arm, 7

said rotary-to-reciprocating motion conversion means being driven by said pawl means when the magnitudes of the periodic displacements of said sensing armchange,

said changes in magnitude of said sensing arm displacement corresponding to axial motion of the edge of said web at said reference point, the change from small to large magnitude indicating the pres ence of said web and the change from large to small magnitude indicating the removal of said web at said reference point.

7. In the improved device for evenly winding a web onto a spool of claim 1, wherein the rotary-toreciprocating motion conversion means further comprises:

a correction cam rotatably mounted on said twistable frame means and intermittently driven by said pawl means,

a cam follower rigidly attached to said machine frame,

said correction cam rotatably mounted within said cam follower,

said twistable frame means undergoing said twist displacement when said correction cam is rotatably driven within said cam follower by means of said pawl means.

8. In the improved device for evenly winding a web onto a spool of claim 1, wherein the pawl means further comprises:

a pawl carrier means,

said carrier means rotatably mounted on said twistable frame means in a position coaxial with and opposed to said ratchet means,

a first pawl, pivotally mounted on said carrier means,

said first pawl being resiliently urged intoengagement with said ratchet means when said ratchet means undergoes clockwise rotation,

a second pawl, pivotally mounted on said carrier means,

said second pawl being resiliently urged into engagement with said ratchet means when said ratchet means undergoes counterclockwise rotation,

'said web axial-position detection means contacting and pivotally displacing said first and second pawls in timed relation with the rotation of said ratchet means,

said displacements causing each of said pawls to avoid engagement with said ratchet.

9. A device for evenly winding a web onto a spool,

comprising:

a fixed machine frame,

a plate pivotally mounted on said machine frame,

a first spool pivotally and rotatably mounted between said plate and said machine frame,

a second spool pivotally and rotatably mounted between said plate and said machine frame,

said plate imparting a relative skew between said first and said second spool when displaced about its pivot point,

a reversible motor mounted on said plate for rotatably driving said first and said second spool in either direction,

a switching means mounted on said plate for reversing the direction of drive of said motor when the end of said web is detected unwinding from either of said spools;

a double pawl clutch rotatably mounted on said plate and continuously driven by said motor;

a mechanical web-edge detection means mounted on said plate,

said detection means periodically actuating said double pawl clutch in timed relation with the rotation of said clutch;

a cam rotatably mounted on said plate,

a cam follower rigidly mounted on said machine frame;

said cam rotatably engaging said cam follower and being intermittently driven by said double pawl clutch when said detection means senses a change in the axial position of said web as it is wound upon either said first or said second spool;

said pivot plate being driven through an angular displacement about its pivot point as said cam is controllably driven into rotation by said double pawl clutch;

said web undergoing a trail-off correction due to the change in the relative skew of the first and second spools as controlled by said detecting means sensing the presence or absence of said web at a reference point substantially midway between the extreme axial displacements of said web;

whereby said web may be evenly wound on either said first or said second spool, automatically. 

1. In combination with a device for evenly winding a web onto a spool of the type comprising a fixed machine frame, a twistable frame means mounted on said machine frame, said twistable frame means capable of assuming either of two states of twist, a first and a second spooling means, said first and second spooling means pivotally and rotatably mounted in said twistable frame means, said two states of twist imparting alternate states of relative skew between said first and said second spooling means, said first spooling means winding said web from said second spooling means and said second spooling means winding said web from said first spooling means, a rotary driving means for rotatably driving said first and second spooling means, a web axialposition detection means, a reciprocating drive means, said reciprocating drive means intermittently actuated by said detection means when a change is detected in the axial-position of said web as it is wound upon either said first or said second spooling means, said twistable frame means driven by said reciprocating drive means so as to assume either of said two states of twist, said web steered into alternate directIons of axial motion on said first or said second spooling means by said alternate states of relative skew, said state of skew controlled by said detecting means sensing the presence or absence of said web at a reference point substantially midway between the extreme axial displacements of said web, wherein the improvement resides in said reciprocating drive means which further comprises: a ratchet means continuously driven by said rotary driving means, a pawl means opposed to said ratchet means and intermittently engagable therewith under the control of said web axialposition detection means, a rotary-to-reciprocating motion conversion means connected to said pawl means, said motion conversion means being intermittently driven by said pawl means when said detection means causes said pawl means to engage said ratchet means, said rotary-to-reciprocating motion conversion means contacting to said machine frame and said twistable frame means so as to impart a relative displacement therebetween when driven by said pawl means.
 2. In the improved device for evenly winding a web onto a spool of claim 1, wherein said web axial-position detection means further comprises: a sensing cam, rotatably mounted on said twistable frame means and continuously driven by said rotary driving means, said sensing cam having at least one lobe, a sensing arm, mounted on said twistable frame means and resiliently urged into engagement with said sensing cam, said sensing arm periodically displaced by the lobes on said cam, through an arc over said reference point, said sensing arm undergoing relatively large displacements when said web is present and relatively small displacements when said web is absent from said reference point,
 3. In the improved device for evenly winding a web onto a spool of claim 2, wherein said reciprocating drive means further comprises: said ratchet means, rotatably mounted on said twistable frame means and continuously driven by said rotary driving means, said ratchet means possessing a number of teeth equal to the number of lobes on said sensing cam, said ratchet means rotating in unison with said sensing cam, a pawl carrier means, said carrier means rotatably mounted on said twistable frame means in a position coaxial with and opposed to said ratchet means, a first pawl, pivotally mounted on said carrier means, said first pawl mounted so as to be resiliently urged into engagement with said ratchet means when said ratchet means undergoes clockwise rotation, a second pawl, pivotally mounted on said carrier means, said second pawl, mounted so as to be resiliently urged into engagement with said ratchet means when said ratchet means undergoes counterclockwise rotation, a resilient detent means mounted on said twistable frame means and positioned so as to stop the rotation of said pawl carrier means at every 180*, said sensing arm periodically contacting and pivotally displacing said first and said second pawls, as it is driven by said sensing cam in timed rotation with the rotation of said ratchet means, said pawl displacement causing said first pawl to avoid engagement with said ratchet means during clockwise rotation of said ratchet and causing said second pawl to avoid engagement with said ratchet during counterclockwise rotation of said ratchet, said sensing arm omitting contact with said first pawl during clockwise rotation of said ratchet or omitting contact with said second pawl during counterclockwise rotation of said ratchet, when a change in the magnitude of said periodic displacements of said sensing arm occurs, said displaced pawl engaging said ratchet means when said change in the magnitude of said sensing arm displacements takes place, said pawl carrier means thereby being driven into rotation, said pawl carrier rotation continuing until said detent means halts said rotation, at which point said sensing arm contacts said engaged pawl, disengaging said pawl, said pawl carrier means remaining stationary in its new state as long as said periodic sensing arm displacements retain their new magnitude.
 4. In the improved device for evenly winding a web onto a spool of claim 3 wherein said reciprocating drive means further comprises: a correction cam rotatably mounted on said twistable frame means and rotating in unison with said pawl carrier means, a cam follower rigidly attached to said machine frame, said correction cam rotatably mounted within said cam follower, said twistable frame means undergoing a twist displacement when said correction cam is rotatably driven by 180* within said cam follower by means of the controlled rotation of said pawl carrier means, said state of relative skew of said first and second spooling means being altered by the controlled displacement of said twistable frame means, said web being steered into alternate directions of axial motion along said first and second spooling means, by the controlled changes in said relative skew, whereby said web may be evenly wound on either said first or said second spooling means, automatically.
 5. In the improved device for evenly winding a web onto a spool of claim 1, wherein the rotary driving means further comprises: a first reversible motor mounted on said twistable frame means, said first motor driving said first spooling means, a second reversible motor mounted on said twistable frame means, said second motor driving said second spooling means, said first motor, when driving said first spooling means so as to wind said web thereupon, being fully energized to advance the web, while said second motor being energized in the direction opposing the advance of the web, at a magnitude of substantially one half that of said first motor, thus imparting a drag force on the web, an end-of-web reversing means for detecting the end of the web before it unwinds from the spooling means supplying said web, said reversing means automatically reversing the driving direction of said first and second motors, said second motor becoming fully energized so as to advance the web in the direction reversed to its preceding motion and said first motor becoming energized at one half its preceding magnitude and in the direction opposed to the new direction of advance for the web, whereby said web remains in tension without regard to its direction of advance.
 6. In the improved device for evenly winding a web onto a spool of claim 1, wherein the web axial-position detection means further comprises: a sensing cam continuously driven by said rotary driving means, a sensing arm mounted on said twistable frame means so as to be resiliently urged into engagement with said sensing cam, said sensing arm being periodically lifted and lowered over said reference point by means of said sensing cam, a collapsible finger means, pivotally mounted on said sensing arm, said finger means located substantially over said reference point, said finger means frictionally engaging said web when present at said reference point, when said cam lowers said sensing arm, said finger means following the advancing motion of said web, when frictionally engaging the web causing the finger means to collapse and the sensing arm to undergo a relatively large displacement, said finger means remaining in a standing position on a supporting surface for said web, when said web is absent from said reference point, when said sensing cam lowers said sensing arm, thereby limiting the sensing arm to a relatively small displacement, said sensing arm periodically actuating said pawl means as said sensing cam lifts and lowers said sensing arm, said rotary-to-reciprocating motion conversion means being driven by said pawl means when the magnitudes of the periodic displacements of said sensing arm change, said changes in magnitude of said sensing arm displacement correSponding to axial motion of the edge of said web at said reference point, the change from small to large magnitude indicating the presence of said web and the change from large to small magnitude indicating the removal of said web at said reference point.
 7. In the improved device for evenly winding a web onto a spool of claim 1, wherein the rotary-to-reciprocating motion conversion means further comprises: a correction cam rotatably mounted on said twistable frame means and intermittently driven by said pawl means, a cam follower rigidly attached to said machine frame, said correction cam rotatably mounted within said cam follower, said twistable frame means undergoing said twist displacement when said correction cam is rotatably driven within said cam follower by means of said pawl means.
 8. In the improved device for evenly winding a web onto a spool of claim 1, wherein the pawl means further comprises: a pawl carrier means, said carrier means rotatably mounted on said twistable frame means in a position coaxial with and opposed to said ratchet means, a first pawl, pivotally mounted on said carrier means, said first pawl being resiliently urged into engagement with said ratchet means when said ratchet means undergoes clockwise rotation, a second pawl, pivotally mounted on said carrier means, said second pawl being resiliently urged into engagement with said ratchet means when said ratchet means undergoes counterclockwise rotation, said web axial-position detection means contacting and pivotally displacing said first and second pawls in timed relation with the rotation of said ratchet means, said displacements causing each of said pawls to avoid engagement with said ratchet.
 9. A device for evenly winding a web onto a spool, comprising: a fixed machine frame, a plate pivotally mounted on said machine frame, a first spool pivotally and rotatably mounted between said plate and said machine frame, a second spool pivotally and rotatably mounted between said plate and said machine frame, said plate imparting a relative skew between said first and said second spool when displaced about its pivot point, a reversible motor mounted on said plate for rotatably driving said first and said second spool in either direction, a switching means mounted on said plate for reversing the direction of drive of said motor when the end of said web is detected unwinding from either of said spools; a double pawl clutch rotatably mounted on said plate and continuously driven by said motor; a mechanical web-edge detection means mounted on said plate, said detection means periodically actuating said double pawl clutch in timed relation with the rotation of said clutch; a cam rotatably mounted on said plate, a cam follower rigidly mounted on said machine frame; said cam rotatably engaging said cam follower and being intermittently driven by said double pawl clutch when said detection means senses a change in the axial position of said web as it is wound upon either said first or said second spool; said pivot plate being driven through an angular displacement about its pivot point as said cam is controllably driven into rotation by said double pawl clutch; said web undergoing a trail-off correction due to the change in the relative skew of the first and second spools as controlled by said detecting means sensing the presence or absence of said web at a reference point substantially midway between the extreme axial displacements of said web; whereby said web may be evenly wound on either said first or said second spool, automatically. 